Transgenic mice with insertional mutations constitute a potentially rich source of molecularly tagged important genes. We have selected two insertional mutations for closer scrutiny. Mice with an insertion on chromosome 11 (map position 58) display a mild developmental abnormality of the vertebrae characterized by vertebral fusions and split vertebrae particularly in the thoracic and lumbar regions. This phenotype is reminiscent of that created by mutations in pax 1, a vertebrate transcription factor, whose gene, however, is on chromosome 2. It is conceivable that the interrupted gene on chromosome 11 is one of the elusive target genes of Pax 1. We have identified a piece of genomic DNA flanking the inserted transgene. The corresponding sequence is expressed on a 2.8 kb mRNA found in adult tissues such as muscle and heart. Currently, we are identifying a second piece of flanking DNA that might allow us to map the extent of any deletion that may have accompanied the insertion. We will then proceed to identify the transcript of the region of chromosome 11 that is responsible for the phenotype. A second insertion occurred into the genetically defined mi locus on mouse chromosome 6. Mutations in mi are characterized by the triad, loss of coat pigment, small red eyes, and hearing deficiency. The common denominator of these defects may be the underdevelopment, or loss, of functional melanocytes throughout the body. In fact, analysis of the inner ear of these transgenic mice has revealed that the so-called stria vascularis is free of melanocytes, a phenomenon that is associated with degeneration of the outer hair cells. We have identified two pieces of genomic DNA flanking the inserted transgenes. These sequences are present on a single genomic fragment of 6.6 kb and define an RFLP in another line of mice with a mutation at mi. One of these probes detects on Northern blots approximately 7.0 kb mRNA found in adult skin and melanocyte cell lines, and not in many other organs. Thus, this mRNA likely represents a transcript of the mi gene. The importance of the analysis of transgenic insertional mutations not only lies in the analysis of developmentally important genes but also in the potential to molecularly characterize human genetic diseases with similar phenomenology. The vertebral abnormality defined by the chromosome 11 insertion is reminiscent of certain human vertebral malformations, and the mi mutation is a model for particular forms of human Waardenburg syndrome characterized by hearing deficiency and pigment abnormalities.